1. FIELD OF THE INVENTION
This invention is directed to a cutting tool and, more specifically, to a cutting tool, such as a drill, having at least one helical flute with an associated land and with a curved heel connecting the flute and land.
This invention is also directed to a method and apparatus for making a cutting tool having at least one helical flute and land. In particular, a grinding wheel may be used to produce both the flute and land portions of the cutting tool.
This invention is also directed to a crush roll used to form such a grinding wheel.
2. DESCRIPTION OF RELATED ART
A cutting tool, such as a twist drill, is generally comprised of a cylindrical shaft having at least one flute and land, which follow a helical angle to a forward cutting edge at the forward end of the shaft. The land has associated with it a margin and a clearance portion behind the margin. A side cutting edge is defined by the intersection of the flute with the margin of the land. In many drills, it is typical for the portions of the land opposite the side cutting edge to terminate in a sharp edge. However, such an edge is not functional during a cutting operation and is a result solely of the manufacturing process, which utilizes a first grinding wheel to form the flute and a second grinding wheel to form the land, including the margin, of the drill. Nevertheless, this sharp edge generally disrupts the flow of any coolant that may be introduced to the drill during the cutting operation and, furthermore, weakens the drill as a result of excess removal of material in forming such an edge and impedes chip flow during a drilling operation which, in turn, limits the depth of penetration for such a drill to approximately 4-5 times the outer diameter of the drill. Deeper depths are possible if the drill is retracted from the hole so that cutting chips may be expelled from the flutes, then reintroduced to the hole to continue drilling. This is known as pecking.
Certain drills do not have this sharp edge. Parabolic drills are generally comprised of a cylindrical shaft having a flute and land. The sharp edge at the intersection of the flute and the land away from the lip is eliminated in favor of a curved edge. Parabolic drills are typically used for low-speed, light-load applications. Parabolic drills are designed for deep penetrations and, as a result, have a relatively high helix angle and have a deep flute to promote chip excavations. The deep flute causes the outer edges of the drill to be relatively thin and this reduces the torsional strength of the drill. Furthermore, the reduction in cross-sectional area at the outer edges of the drill causes the drill to be more prone to bending and buckling. However, as a result of this design, a parabolic drill is typically capable of producing a hole within a workpiece having a depth between 10-15 times the outer diameter of the drill without the need to peck.
Soviet Patent No. 948624, titled xe2x80x9cMethod for Manufacture of Twist Drillsxe2x80x9d, in FIGS. 1-3, illustrates a twist drill, wherein the intersection between a flute and land opposite the side cutting edge is a curved section. However, the small radius of the curved section provides it with a relatively shallow flute and other features most resembling the conventional drill previously discussed. Coolant flow is still constrained by the land, and material must be undesirably removed from the center of the drill to accommodate the sharp curvature at the intersection between the flute and the heel, thereby reducing the core diameter of the drill.
A twist drill design is desired which permits improved coolant and chip flow while simultaneously maximizing the drill core diameter, thereby maximizing torsional strength of the drill and providing a drill having the ability to penetrate a workpiece to a depth of greater than 4-5 times the drill outer diameter.
Additionally, in the past, production of a cutting tool having both a helical flute and a land with a margin required two separate grinding operations. In one operation, the flute would be ground with one grinding wheel, and in a separate operation, the land would be ground with a second grinding wheel.
Additionally, these two separate grinding operations require mounting the cutting tool on two separate grinding machines. Therefore, the cutting tool must be precisely mounted within each grinding machine to ensure the flute and land are ground along the same helical path. Additionally, the grinding machines must be set at the same lead angle to ensure the flute and land are ground along the same helix angle. If the cutting tool is not properly mounted and the flute and land are not ground correctly, the tool may be flawed and, as a result, must be discarded. Furthermore, setup for two separate operations is not only time-consuming but also prone to error. Therefore, while the cutting tool could be ground using two grinding operations, a single grinding operation would be preferred.
A need exists to eliminate the need for two separate grinding operations through the use of a single grinding wheel and a method for using the same.
Finally, such a grinding wheel may be produced using single point dressing tools or CNC machines. However, each of these methods is time-consuming and expensive. As a result, a method and apparatus is desired for producing such a grinding wheel in a more efficient manner.
One aspect of the subject invention is directed to a cutting tool has a shaft with a longitudinal axis, wherein the shaft has a forward end and the shaft has an external surface. The external surface, when viewed in a cross-section perpendicular to the longitudinal axis and spaced from the forward end, has a flute extending about the longitudinal axis of the shaft along the helix angle, wherein the flute has a lip extending inwardly from an outer radius and a nose adjacent to the lip. The external surface also has a land adjacent to the flute and extending about the longitudinal axis of the shaft along the helix angle. The land has a margin which is adjacent to the lip and has a radius equal to the outer radius. The land also has a clearance portion which is adjacent to the margin and has a radius less than the outer radius. A convex heel connects the nose with an adjacent clearance portion to provide a continuous surface between the web and clearance portion. The heel preferably has a radius of between 15-75% of the outer radius. The smallest width of the tool at the centerline is less than the peripheral distance along any clearance portion.
Another aspect of the subject invention is directed to a grinding wheel for producing a helical cutting tool with a shaft with a longitudinal axis. The cutting tool shaft has a forward end and is comprised of a flute having a radial depth and extending about the longitudinal axis of the shaft along a helix angle and a land adjacent to the flute. The flute has a lip extending inwardly from an outer radius and a nose adjacent to the lip. Each land has a margin adjacent to the lip and has a radius equal to the outer radius and a clearance portion adjacent to the margin, wherein the clearance portion and the nose are connected by a heel. The grinding wheel is comprised of a disk having a disk centerline and an average radial size, a first side and a second side defining a width therebetween, and a disk outer edge between the sides, wherein the edge has an abrasive material for grinding a cutting tool and wherein the edge has a profile defined by:
a) a lip shaper segment having a convex shape and extending in a lateral direction from the first side and extending away from the disk centerline a radial distance equal to the radial depth of the flute;
b) a nose shaper segment adjacent to the lip shaper segment and extending in a lateral direction toward the second side and extending inwardly toward the disk centerline;
c) a clearance shaper segment having a curved shape and extending from the nose shaper segment toward the second side and having a radius greater than eight times the tool outer radius; and
d) a margin shaper segment extending from the clearance shaper segment toward the second side and inwardly toward the disk centerline.
Yet another aspect of the subject invention is directed to a method of producing a cutting tool having a shaft with a longitudinal axis and having a flute with a lip and a nose and an adjacent land with a clearance portion and a margin. The flute and land are along a helix angle, the margin has an outer radius, and the clearance portion is recessed from the margin. The method comprises the step of:
a) producing both the flute and land of the cutting tool by grinding the shaft with a single pass of a grinding wheel, wherein the grinding wheel has a clearance shaper segment to produce a clearance portion on the cutting tool, and wherein the clearance shaper segment has a radius of greater than ten times the outer radius.
Another aspect of the subject invention is directed to a cutting tool having a shaft with a longitudinal axis having a flute with a lip and a nose, and an adjacent land with a clearance portion and a margin. The flute and land are along a helix angle, the margin has an outer radius, and the clearance portion is recessed from the margin. The cutting is produced with a method comprising the step of:
a) producing both the flute and land of the cutting tool by grinding the shaft with at least one pass of a single grinding wheel, wherein the grinding wheel has a clearance shaper segment to produce a clearance portion on the cutting tool, and wherein the clearance shaper segment has a radius of greater than ten times the outer radius.
A final aspect of the subject invention is directed to a crush roll for dressing a grinding wheel, wherein the grinding wheel is used to produce a helical cutting tool with a shaft with a longitudinal axis, wherein the cutting tool shaft has a forward end and is comprised of a flute having a radial depth and extending about the longitudinal axis of the shaft along a helix angle and a land adjacent to the flute. The flute has a lip extending inwardly from an outer radius and a nose adjacent to the lip. Each land has a margin adjacent to the lip and has a radius equal to the outer radius and a clearance portion adjacent to the margin, wherein the clearance portion and the nose are connected by a heel. The crush roll is comprised of a disk having a disk centerline and an average radial size, a first side and a second side defining a width therebetween and a disk outer edge between the sides, wherein the edge has an abrasive material for dressing the grinding wheel and wherein the edge has a profile defined by:
a) a lip former segment extending in a lateral direction from the first side and extending toward the disk centerline, a radial distance equal to the radial depth of the flute;
b) a nose former segment adjacent to the lip former segment and extending in a lateral direction toward the second side and extending outwardly from the disk centerline;
c) a clearance former segment having a curved shape and extending from the nose former segment toward the second side and having a radius greater than eight times the outer radius of the cutting tool; and
d) a margin former segment extending from the clearance former segment toward the second side and extending outwardly from the disk centerline.